Orbital motion and satellites
Orbits (Separate Physics only)
provides the force that keeps objects in :
- the orbit the Sun
- the Moon orbits the Earth
- orbit the Earth
Most orbits are close to circular.
Artificial satellites are used for communications, weather forecasting, satellite navigation and observing the Earth. satellites orbit once every 24 hours above the equator, so they stay above the same spot on the ground. That's useful for TV and communications.
Key terms
Speed and orbit radius (Higher tier only) (Separate Physics only)
In a circular orbit, gravity pulls the object towards the centre, at right angles to its motion. This changes the object's direction but not its . So its keeps changing while its speed stays the same: it is accelerating towards the centre.
For a stable orbit, the radius must change if the speed changes:
- Closer to the planet, , so an object in a must move faster to stay in orbit rather than fall inwards.
- An object in a larger orbit moves more slowly.
For example, the International Space Station orbits about 400 km up and goes round in about 90 minutes. Geostationary satellites are about 36 000 km up and take 24 hours.